Love BIAB! But I will be going to a single-tier system when I move. Too limited on space in my current home.
From my experience on 12+ batches now of BIAB:
-You need a 10 gal+ kettle to brew 5.5 gallon batches
-The bag is not fun to clean - I think a cooler would actually be easier IMO
-I get a lot of trub in the bottom of the kettle/fermentor, hence 5.5gal final volume batches
-I have to double-mill my grains to maintain 70% efficiency
I don't have trouble with mash temps. I use a camp stove 60,000 BTU. I keep the lid on and kick the burner on for a couple minutes if the temp drops and then I mashout to 160 occasionally. Not perfect, but not hard either.
I reemphasize the 10gal+ kettle. Typical water requirements are 8-9gal for what I usually brew (Pale Ales & IPAs) with up to 13lbs of grain. My 10gal kettle is usually at the brim when douging in, so much that I have to dough in slowly to ensure it doesn't overflow. No sparge, just thinner mashes here around 1.5-1.75 qt/lb.
Got the space? Jump on board with the Brewers Buddy and go with a tiered brewing system!
https://www.kickstarter.com/projects/1100391693/brewers-buddy-your-best-friend-in-brewing
Sorry - had to plug that product as I really want it to get funded for my own gain hahaha